This invention relates to electrodeless low pressure mercury rare gas discharge lamps. Such devices comprise a glass lamp vessel filled with a rare gas or rare gas mixture and a mercury or other vapor dose, then sealed under vacuum conditions. The walls of the vessels are partially coated with a high reflectivity material such as Al.sub.2 O.sub.3 on top of which a layer of phosphor mix is deposited. An external spiral coil is used to initiate and maintain a discharge in the lamp. Electrodeless low pressure mercury rare gas discharge lamps containing a phosphor layer have been disclosed before as long life fluorescent lamps.
1. References:
(a) J. M. Anderson, "High Frequency Electrodeless Fluorescent Lamp Assembly", U.S. Pat. No. 3,521,120 of Jul. 21, 1970; (b) J. M. Anderson, "Electrodeless Fluorescent Lamp Bulb RF Power Energized Through Magnetic Core Located Partially Within Gas Discharge Space". U.S. Pat. No. 3,987,335 of Oct. 19, 1976; (c) D. D. Hollister, "Light Generation by an Electrodeless Fluorescent Lamp", U.S. Pat. No. 4,010,400 of Mar. 1, 197; (d) J. M. Proud and R. K. Smith, "Compact Fluorescent Light Source Having Metallized Electrodes", U.S. Pat. No. 4,266,166 of May 5, 1981; (e) H. Houkes, J. W. Denneman, P. Postma, "Discharge Lamp With Interference Shielding", U.S. Pat. No. 4,568,859 of Feb. 4, 1986; (f) H. Houkes, P. Postma, and A. C. van Veghel, "Electrodeless Low Pressure Discharge Lamp". U.S. Pat. No. 4,710,678 of Dec. 7, 1987; and (g) P. Postma and A. C. van Veghel, "Electrodeless Low Pressure Discharge Lamp". U.S. Pat No. 4,727,295 of Feb. 23, 1988. Many of these lamps comprise an incandescent light bulb which has a coil of some number of turns either inside or outside the bulb. The operating frequency of such a lamp typically is in the MHz range with many of them operating at 13.56 MHz. These lamps typically are distinguished by the fact that they tend to be longer living compared to electroded-version-compact or otherwise fluorescent devices and they need some provision to reduce the electromagnetic interference (EMI) to levels acceptable to the various regulatory agencies around the world. None of these disclosures, however, anticipate a use where the lamp is as thin as possible, and it gives off light in a specific direction for increased effectiveness of illumination.
In the prior art the inductively coupled plasma generated by a solenoid coil forms a ring with the azimuthal symmetry but strongly non-uniform in the radial and axial directions. The bulb typically has a shape of the incandescent lamp and has a spherical symmetry. As a result, the light illumination is predominantly in the radial direction, while the light illumination through the top and the bottom of the bulb is much weaker. This radial and axial light illuminance non-uniformity is the inherent feature of the lamp described in prior patents cited herein (at n. 1) and makes these types of lamp somewhat less than optimum for directional light illumination.
The coil inserted inside the reentrant cavity is subjected to the extensive heat from the plasma generated at the adjacent walls. This heating of the coil results in the increase of its temperature and hence in the rise of the coil resistance. This causes an increase of RF power losses in the coil, P=I.sup.2 R, that reduces lamp efficiency. The thermal management of the coil is one of the major problems in the design of lamps with high light illumination (greater than 3000 lumens) which requires RF power, P&gt;50 W.
Prior art electrodeless lamp disclosures (including those cited at n. 1) describe lamps with reentrant cavities employ a bulb shape close to the classic incandescent lamp bulb shape. As a result of this design, the distance between the wall of the reentrant cavity is a few centimeters. This is a relatively long path for the mercury resonance line generated primarily near the outer walls of the reentrant cavity. Such a path results in resonance line trapping and hence reduces the radiation efficiency.
It is an objective of this invention to provide an electrodeless flat lamp energized inductively with a spiral shaded coil adjacent to one of the surfaces of the lamp and coupling RF power efficiently to the lamp gas mixture.
It is another objective of this invention to provide a flat or substantially flat lamp with uniform illumination in such a manner that it requires either a very small and inexpensive fixture or no fixture at all for general illumination, either in downlights or sconce (wall) lighting or some other accent lighting.
Another objective is to provide an efficient substantially flat lamp with as little resonance trapping as possible so as to increase the brightness density across its surface.
A further objective is to provide as compact a package as possible by having a very thin lamp that has uniform brightness.
Yet a further objective is to provide a lamp whose emission is mostly directed in one dimension thereby increasing its effectiveness in illuminating that particular dimension.
A further objective is to provide an electrodeless light source in which the spatial distribution of light can be altered at will, in any given direction.